Such a method is to be found, for example, in communication controllers of the Controller Area Network (CAN). It is discussed in the BOSCH CAN-Specification 2.0, for instance, which can be downloaded from the Robert Bosch GmbH website http://www.semiconductors.bosch.de. The bus system is usually a wiring pair such as a twisted copper cable. The CAN protocol is widespread in the automotive industry, industrial automation or in building networking, for instance. The messages to be transmitted in the CAN protocol have a header section, a data field and a trailer section, the data to be transmitted being contained in the data field. The header section of the message contains a start-of-frame bit, an arbitration field, as well as a control field. The arbitration field includes the identifier, which determines the priority of the message. CAN supports identifier lengths of 11 bits (“standard format” or “base format”) and 29 bits (“extended format”). The control field includes a data length code, which specifies the length of the data field. The trailer section of the message has a CRC field, an acknowledge field and an end-of-frame field. This CAN protocol is referred to hereinafter as “Norm CAN”. Bit rates up to 1 Mbit/s are reached via the Norm CAN.
The roles of transmitter and receiver for the messages to be transmitted are assigned among the nodes by an arbitration process based on information from the header section of the messages. In this context, arbitration process means that based on an identifier contained in the message, it is negotiated which node will receive transmit access to the bus when several nodes are attempting simultaneously to transmit a message, in doing which, given unambiguously assigned identifiers, the transmit access is awarded by the arbitration process to exactly one node. In the case of CAN, the at least one control bit assumed for our invention is contained in the header section and, for example, is a reserved bit in the arbitration field or in the control field that, for instance, must always be transmitted dominantly with a predetermined value.
Many other communication systems recognize similar reserved bits, which are always transmitted with a fixed value. Hereinafter, the inventive idea is described based on the CAN. However, the invention is not thereby limited to CAN bus systems, but rather may be carried out starting from all bus systems that satisfy the features according to the definition of the species of the method claimed.
The introduction of ever more highly networked applications, e.g., of assistance systems in vehicles or of networked control systems in industrial facilities, leads to the general demand that the bandwidth for serial communication must be increased.
Two factors limit the effective data transfer rate in Norm CAN networks, namely, on one hand, the bit duration, thus, the length of one bit in time, which is limited downward by the function of the CAN bus arbitration process, and on the other hand, the relationship between the number of data bits and control bits, thus, bits not containing useful data, in a CAN message.
Another protocol is known which is called the “CAN with flexible data rate” or CAN FD. It uses the bus arbitration process familiar from the CAN, but increases the bit rate by switching over to a shorter bit duration after the end of the arbitration up to the bit CRC delimiter. In addition, the effective data transfer rate is increased by allowing longer data fields. The CAN FD is also a method for the exchange of data between nodes that are connected to each other by a bus system, in which the messages that contain the data are exchanged according to a first communication protocol, the messages are made up of a sequence of bits and within each message that contains data, at least one control bit having a predetermined position within the sequence of bits must have a predetermined value.
CAN FD may be used for general communication, but also in certain operating modes, e.g., for software download or end-of-line programming or for maintenance work.
CAN FD requires two sets of bit-timing configuration registers which define one bit duration for the arbitration phase and a further bit duration for the data phase. The bit duration for the arbitration phase has the same restrictions as in the Norm CAN networks; the bit duration for the data phase may be selected to be shorter in view of the handling capacity of the transceivers selected and the requirements of the CAN FD network.
The transceivers or bus-interface units are assigned the task in the network of converting the logic signals of the communication controller according to the physical link layer provided, to corresponding physical signals on the specific transmission medium. Often, e.g., for CAN as well, the logic signals are represented by generating and transmitting suitable voltage differences as physical signals. This is explained in the following for illustration, using the CAN as example.
For CAN, the logic signals “0” and “1” are usually represented as voltage difference between the two as a rule metallic (e.g., copper) lines of the bus system. In this context, for the representation of a “0”, the transceiver usually actively sets a predetermined first differential-voltage level of, e.g., 2 volts by, for instance, allowing a current to flow with the aid of a suitable current source, so that the desired voltage difference ensues. This driven first differential-voltage level cannot be overwritten by another bus node. The level and the corresponding bus state are therefore denoted as “dominant”.
To represent a “logic 1”, the current is not further driven. A current flows across one or more terminating resistors which are provided at the ends of the bus line between the two wires of the bus system, for example, so that a second differential-voltage level ensues, which corresponds to logic “1”. This second differential-voltage level may be zero, but may also be set by suitable voltage sources to a value other than zero. This ensuing second differential-voltage level is able to be overwritten by another bus node with a dominant level. The second level and the corresponding bus state are therefore denoted as “recessive”.
In addition, the transceiver continuously ascertains the voltage difference between the two lines, so as by a comparison to threshold values, for example, to determine whether a dominant or recessive bus level exists at the moment.
A CAN FD message is made up of the same elements as a Norm CAN message, which differ from each other in detail, however. Thus, the data field and the CRC field may be longer in a CAN FD message. Examples for Norm CAN messages and CAN FD messages are illustrated in FIG. 1.
CAN FD supports both identifier lengths of the CAN protocol, the 11-bit long “standard format”, also called “base format”, and the 29-bit long “extended format”. CAN FD messages have the same structure as Norm CAN messages. Norm CAN messages are distinguished from CAN FD messages by a reserved bit which, in the Norm CAN, is always transmitted dominantly, bears the name “r0” or “r1” and is located in the control field before the data length code. In a CAN FD message, this bit is transmitted recessively and is called EDL. In comparison to Norm CAN messages, additional control field bits follow in the CAN FD messages, for example, the bit BRS which indicates the position at which—provided the BRS bit has a corresponding value—the bit duration in a CAN FD message is switched over to a shorter value. This is represented in FIG. 1 by arrows, which split the messages into a section having the designation “CAN FD data phase”, in which the high bit rate or the short bit duration is used, and into two sections having the name “CAN FD arbitration phase”, where the lower bit rate, i.e., the longer bit duration is used.
The number of bytes in the data field is indicated by the data length code. This code is 4 bits in size and is transmitted in the control field. The coding in the case of CAN FD is different than in the Norm CAN. The first nine codes (0x0000 to 0x1000) are the same, but the following codes (0x1001 to 0x1111) correspond to larger data fields of the CAN FD messages, e.g., 12, 16, 20, 24, 32, 48 and 64 Bit.
Norm CAN transceivers may be used for CAN FD, special transceivers are optional, and in some instances, may contribute to a further increase of the bit rate in the data phase.
The CAN FD protocol is described in a protocol specification having the title “CAN with Flexible Data-Rate Specification”, referred to hereinafter as the CAN FD Specification, which can be downloaded on the Robert Bosch GmbH website M http://www.semiconductors.bosch.de.
So long as unmodified Norm CAN controllers are used, a mixed network of Norm CAN nodes and CAN FD nodes are only able to communicate in the Norm CAN format. That is, all nodes in the network must have a CAN FD protocol controller in order to carry out CAN FD communication. However, all CAN FD protocol controllers are capable of participating in Norm CAN communication.
One reason for this fallback on the slower communication in mixed networks is the monitoring of the communication by the communication nodes, which is partially responsible for the high transmission reliability in CAN bus systems, for example. Since the unmodified Norm CAN controllers are unable to correctly receive the faster data bits of the CAN FD messages, they would destroy these messages by error messages (so-called error frames). Similarly, CAN FD controllers would destroy messages by error frames they would attempt to transmit after arbitration has taken place, e.g., using a bit duration once again shortened with respect to the CAN FD specification or using another bit coding or a deviating protocol. Thus, in general, the transmission rate may be limited by one of the slower nodes in the network, or rather, by its monitoring mechanism.
Particularly when data is to be transmitted between two specific nodes that are set up for a differing, e.g., faster communication protocol, this limitation is not always necessary and may be disadvantageous, especially when it is possible to dispense with the monitoring mechanisms, which would lead to destruction of the differing or faster messages, in the case of this differing communication protocol.
Thus, at least in certain application cases, substantially higher transmission rates may be attained if the monitoring mechanism which brings about the destruction of the messages by error frames is interrupted by suitable mechanisms under certain conditions.